1. Field of the Invention
This invention relates to a thermal transfer printing device and method for thermally transferring thermal transfer inks to a recording medium to produce images on the recording medium by exothermically driving a thermal print head, and more particularly to a thermal transfer printer and method capable of rationally moving and accurately positioning a card-like recording medium at a printing position.
2. Description of the Prior Art
In a printer for producing images using an ink ribbon applied with a thermal transfer ink to be thermally transferred to a recording medium by exothermically driving an array of heating resistance elements of a thermal print head, the quality of reproduced images depends upon the accuracy of positioning the recording medium relative to the thermal print head located at a printing position.
For example, when producing colored patterns and/or photographs on a recording medium such as a credit card and an identification card by use of three primary color inks of yellow, magenta and cyan, it is necessary to move the card back and forth relative to the printing position to perform a single colored printing three times. Therefore, by severely controlling the movement of the card so as to accurately position the card at the printing position, high-quality multi-colored images can be produced.
One example of the conventional thermal transfer printers is disclosed in Japanese Patent Application Public Disclosure No. HEI 3(1991)-275362(A). As schematically illustrated in FIG. 1, this prior art printer performs printing by the steps of first feeding a card C as a recording medium from a card stacker 1 onto a moving bed 2 located in a printing portion, moving down and bringing a thermal print head 3 into contact with the card C through an ink ribbon 4 having a thermal transfer ink, and exothermically driving the thermal print head 3 to thermally transfer the thermal transfer ink on the ink ribbon 4 while moving the card C on the moving bed 2 relative to the print head 3, thereby producing images on the card C.
In this prior art printer, during the course of printing, the moving bed 2 carrying the card C is moved by rotating a feed screw 2a along guide rods 2b in the sideways direction d2 perpendicular to the direction d1 in which the card C is transported from the card stacker 1 to the printing portion. Accordingly, this prior art printer necessitates a card transporting mechanism including the card stacker, a card moving mechanism for reciprocating the bed 2 in the sideways direction, and a head driving mechanism for vertically moving the thermal print head several times to produce one colored print output, thus resulting in a large overall size.
Another version of the printer further requires card holding means for securing in position the card C on the moving bed 2 as mentioned above, as typically proposed by Japanese Patent Application Public Disclosure No. HEI 3(1991)-278976(A). Installation of such card holding means 6 as indicated by imaginary lines in FIG. 1 adds to the size and complexity of the printer.
The aforementioned prior art printers each are provided on the card discharge side thereof with a card coating unit 7 for finally coating the card surface with a transparent plastic layer 8. The plastic layer 8 is fusible and thermally transferred to the card surface by moving downward and exothermically driving a thermal print head 9 in contact with the card C through the plastic layer 8. In this card coating unit 7, there is no necessity for accurately positioning the card C relative to the thermal print head 9 because the transparent plastic layer 8 may be roughly stuck on the card surface.
The printer for producing high-quality multi-colored images is required to be highly controlled to position the card relative to the print head on the order of several micron meters corresponding to the size of each of dots or pixels forming the images. However, a driving system for moving the card relative to the print head usually comprises mechanical elements including gears, feed screws, pinions, racks and/of toothed belts. Which inevitably bring about backlash causing obstruction to the accurate positioning of the card relative to the print head. The idea of eliminating such minute mechanical clearance of the mechanical elements to the fullest possible extent has in no way been embodied in conventional thermal printers.